Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $19.64   9.299%   $21.4663   $21.47   $21.45   $21.50  
  Purchase #2 $202.99   12.251%   $227.8583   $227.86   $227.85   $227.90  
  Purchase #3 $84.65   9.340%   $92.5563   $92.56   $92.55   $92.60  
  Purchase #4 $167.98   8.843%   $182.8345   $182.83   $182.85   $182.80  
  Purchase #5 $232.85   4.170%   $242.5598   $242.56   $242.55   $242.60  
  Purchase #6 $25.45   12.645%   $28.6682   $28.67   $28.65   $28.70  
  Purchase #7 $111.35   9.350%   $121.7612   $121.76   $121.75   $121.80  
  7 purchase totals = $844.91       $917.7046   $917.71   $917.65   $917.90